1. Field of the Invention
The present invention relates to a blast furnace taphole drill of the type with hammer action and air-cleaning used for drilling a taphole in the lining of a blast furnace. Drills according to this invention have a flaring head, the end face of which is provided with radial cutting edges formed by the intersection of flat faces which are angled to each other. An axial central passage extends through the head for the cleaning air.
2. Description of the Prior Art
In blast furnace operation, molten pig iron is tapped off at intervals by drilling a taphole in the lining of the furnace, the taphole being about 50 to 60 mm wide and, depending on the construction of the furnace, about 2 to 3 meters long. Once sufficient pig iron has been tapped from the furnace, the taphole is re-closed by packing in a refractory compound which when heated by heat from the furnace, hardens to form a monolithic structure with the area surrounding the hole. For subsequent tapping, a new taphole is drilled at the same site. Tapping time is dependent on the construction of the taphole drill, the type of blast furnace, the number of tapholes per furnace and on the manner in which the furnace is operated. For a medium sized furnace, a tapping time of about 30 minutes is typical.
When drilling with a taphole drill of the type described above, the hammer action is largely applied to the refractory lining, which becomes crushed in places as a result. In addition, a slow rotation is applied to the drill, so that each successive impact is applied to the refractory material in a different place. At the same time compressed air at a pressure of, for example, six atmospheres is delivered via the axial passage through the drill in order to remove crushed refractory material from the hole being formed.
Although the taphole drill can be operated under a variety of conditions, a hammer action of about 30 impacts per second and a drill rotation speed of a few revolutions per second is normal. This permits drilling of a 21/2 meter taphole in 10 to 12 minutes. However, there is a risk that the drill jams, and this means that it has to be withdrawn from the hole and replaced. Jamming of the drill is generally caused by blockage of the air passage or the hole formed by the drill in the vicinity of the drill head with refractory dust. This type of blockage makes removal of the dust impossible and causes the drill to run hot.
Taphole drills and the drive shaft are generally made in one piece, although designs are known where a removable head is bolted to a shaft. Because of the high temperatures at which the drill has to operate, especially towards the end of the drilling procedure, it is not practical to construct the drill from very high-grade and expensive materials. On the contrary, commercial grade steel with a tensile strength of 50 kg/mm.sup.2 is generally used for this purpose.
The problem of ensuring flow of the cleaning fluid has been considered in drills designed for other purposes. U.S. Pat. No. 3,605,924 discloses a drill having cutting inserts in which axially extending drainage grooves are provided at the periphery of the head and branch passages from the central air passage open into these drainage grooves. FR No. 978,749 discloses a similar construction, as does U.S. Pat. No. 2,368,512 for a drill used for cutting rock and having cutting edges in one piece with the drill head. Also of interest is U.S. Pat. No. 4,047,514 which relates specifically to a furnace taphole drill.
In a drill designed for drilling earth formations, having a flat end face with a large number of domed inserts protruding from it, GB No. 1,465,560 shows three shallow flushing grooves extending across the end face from the end of an air passage. Two of these grooves are not radial, since the end of the air passage is eccentric.